cfg80211: select CRYPTO_SHA256 if needed

When regulatory database certificates are built-in, they're
currently using the SHA256 digest algorithm, so add that to
the build in that case.

Also add a note that for custom certificates, one may need
to add the right algorithms.

Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>